1. Field of the Invention
The present invention relates to the processing of workpieces such as semiconductor wafers, and in particular to a system for preventing contaminants and particulates from coming into contact with a back side of a workpiece as the workpiece is vacuum held on a chuck or robotic end effector.
2. Description of Related Art
During the fabrication of semiconductor wafers, workpieces such as wafers, reticles and flat panel displays are transported between various tools in the wafer fab. These tools include process tools for forming the integrated circuit patterns on the wafers, metrology tools for testing the wafers, sorters for sorting and rearranging the wafers within one or more SMIF pods, and stockers for large scale storage of the SMIF pods.
The processes carried out in each of the above tools must be carried out in a relatively contaminant- and particulate-free environment, such as cleanroom class 1 or better. In present day semiconductor fabrication, particles which range from below 0.02 microns (xcexcm) to above 200 xcexcm can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half xcexcm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 xcexcm substantially interfere with 1 xcexcm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 xcexcm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of even greater interest.
Tools within a wafer fab typically include a front end interface which houses components that facilitate and monitor the transfer of wafers and other workpieces between workpiece-carrying pods and the tools. A conventional front end unit generally includes a workpiece handling robot 24 mounted within the housing and capable of r, xcex8, z motion to transfer workpieces between the workpiece carriers, tool and other front end components. The robot includes an end effector capable of holding and transferring workpieces. Such end effectors may include a vacuum gripping system wherein a sealed evacuation compartment beneath a workpiece on the end effector is evacuated, causing atmospheric pressure to press down on the substrate surface. Vacuum holding of workpieces on the end effector results in greatly increased frictional force between the end effector and workpiece.
In order to evacuate the evacuation compartment beneath the workpiece, a flexible hose is threaded through the various links of the robot having a proximal end connected to an electric valve adjacent to or remote from the base of the robot. The hose includes a distal end connected to the evacuation compartment. The valve is in turn connected to both a pump and a source of venting air so that, under the direction of a controller monitoring the transfer of workpieces by the robot, air may be evacuated from the evacuation compartment to hold the workpiece, and air may be vented, or backfilled, to the evacuation compartment to free the workpiece.
In addition to a robot, a tool front end unit generally includes one or more prealigners for performing the operation of wafer center identification, notch orientation, and indicial mark reading. A prealigner includes a rotating chuck for receiving a wafer. The robot deposits the wafer on the chuck, and the chuck then rotates the wafer so that, with the assistance of sensors and/or cameras positioned adjacent the workpiece, the indicial mark may be located and read. Tools, for carrying out workpiece processing, metrology and sorting, may similarly include workpiece holding chucks wherever it is desired to support a wafer or other workpiece in a fixed and repeatable position.
The prealigner and tool workpiece-support chucks may typically also include a vacuum holding system where a sealed area behind the workpiece is evacuated to improve the frictional force holding the workpiece on the chuck. As with a robotic end effector, a hose is connected to a central opening of a shaft supporting the chuck. The central opening is in turn in communication with an evacuation compartment beneath a workpiece seated on the chuck. An electric valve as described above is additionally provided for controlling evacuation and venting to the compartment beneath the workpiece.
For rotating chucks, a seal block 20 as shown in Prior Art FIG. 1 is provided to communicate the evacuation or venting from the valve to an evacuation compartment 22 in a chuck 24. Seal block 20 includes bearings 26 for rotationally supporting the chuck 24, and a bore hole 28 having a first end to which the hose 30 is connected. The bore hole includes a second end open to an annular cavity 32. The chuck-supporting shaft 34 in turn includes a second bore hole 36 which maintains a central opening 38 in the shaft in communication with the annular cavity 32 so that the vacuum/vent may be communicated to the compartment 22 as the chuck rotates. The seal block 20 includes a pair of O-rings 40 above and below the annular cavity 32 to prevent leakage into or out of the annular cavity 32 as shaft 34 rotates.
When a vacuum held workpiece is transferred from an end effector to a support surface, or from a chuck to a robot, it is important that the vacuum be fully vented before the transfer occurs. For example, in the case of the transfer of a vacuum held workpiece from an end effector, if the vacuum is not completely vented when the workpiece is lowered onto the support surface, excessive forces may be generated between the support surface, workpiece and end effector before the seal is broken. These excessive forces can cause the workpiece to bounce, and can scratch, damage or in a worst case, break the workpiece. Extraneous forces on the end effector may also have hysteresis and other adverse effects on the robot. In the same manner, excessive forces may be generated between the support surface, workpiece and end effector if the vacuum is not completely vented in the case of a transfer of a vacuum held workpiece from a chuck.
In order to ensure a vacuum is completely vented, it is therefore conventional practice for the electric valve to simply open the vent port so that air blasts back into the evacuated chamber at the time of workpiece transfer. There are however, several problems associated with this method of venting. First, the venting source of air brought in through the venting orifice is typically from the fab itself, and is significantly less clean than the cleanroom class 1 or better conditions in which workpieces such as wafers are typically handled. Second, the rapid blasting of air back into the evacuation chamber can dislodge particles from within the electric valve or within the hose. Additionally, in the case of a rotating chuck, where the stationary O-rings 40 (FIG. 1) lie in contact with the rotating shaft 34, particlulates from the O-rings may also be a source of contaminants that are dislodged upon venting.
While it would be possible to delay the workpiece transfer until the vacuum is completely vented, there are literally thousands of such transfers that occur every day in a semiconductor wafer fab, and the cumulative effect of such a delay would be significant.
The particles and contaminants that are brought in through the venting orifice and particles and contaminants that are dislodged upon the rapid venting are communicated up to the evacuation chamber and into contact with the back side of the wafer. When the wafer is subsequently loaded into a wafer-carrying pod, these contaminants can dislodge and fall onto the upper surface the wafer below. Moreover, it is currently common practice to polish the back side of wafers to provide a highly controlled thickness of a wafer and to provide a precisely defined reference surface. Having particulates contact the back side of wafers significantly undermines the advantages that back side polishing provides.
It is therefore an advantage of the present invention to prevent the introduction of particulates and contaminants onto the back side of workpieces including semiconductor wafers, reticles and flat panel displays.
It is a further advantage of the present invention to allow rapid venting of air to fully dissipate a vacuum prior to transfer of a workpiece.
It is another advantage of the present invention that it may be quickly and easily incorporated and maintained within a conventional end effector or workpiece chuck.
These and other advantages are provided by the present invention which in preferred embodiments relates to a system for preventing contaminants and particulates from coming into contact with a back side of a workpiece as the workpiece is vacuum held on a chuck or robotic end effector. The vacuum system according to the present invention comprises a filter provided along the vacuum path, and a novel configuration of an evacuation channel in the workpiece support surface for the vacuum holding of workpieces on the support surface.
The filter serves at least three functions. First, as vented air blasts into the vacuum path, the filter removes particles dislodges by the venting air so that only clean air or other vented gas comes into contact with the workpiece. Second, the filter impedes the flow of air to the back side of the wafer. Thus, particles in the area of the workpiece are not dislodged. Flow may be controlled by the filter by controlling the area of the filter and/or the type of filter. Moreover, by positioning the filters near to the distal end of the vacuum path, i.e., at or near the wafer holding area, the blasted vent air may travel rapidly over the majority of the vacuum path, and have only a minimal volume to backfill downstream of the filter. This minimal volume may be filled relatively quickly, even at the reduced flow rate, so that no delay occurs in venting the vacuum.
In a further aspect of the invention, the workpiece support surface on either the end effector or workpiece chuck includes an evacuation channel for communicating the lower pressure to the back side of the workpiece. By making the evacuation channel relatively shallow, this reduces the volume of air required to vent the evacuation channels. The evacuation channel is defined by a pair of walls which contact the back side of the workpiece. These walls preferably have a minimal width, thus reducing the likelihood of particulate generation as a result of contact between the workpiece and support surface.